1. Field of the Invention
The present invention relates to a switch matrix circuit, a logical operation circuit, and a switch circuit.
2. Description of Prior Art
The FPGA (field programmable gate array) is known as a gate array that is programmable in the state of being implemented. FIGS. 10A and 10B show switch circuits used in the conventional FPGA.
The switch circuit 1 shown in FIG. 10A is provided with a transistor 5 and a cross-coupled flip-flop 3 connected to the gate terminal of the transistor 5. Gates (not shown) for constituting logic are connected to the input terminals 7 and 9 of the transistor 5.
The gates for constituting the logic may be connected to or disconnected from each other by turning on or off the transistor 5 according to the stored content (switching data) in the cross-coupled flip-flop 3. Since the data stored in the cross-coupled flip-flop 3 are rewritable, connection or disconnection between the logic constituting gates may be changed for any number of times.
The switch circuit 11 shown in FIG. 10B is provided with a floating gate type of transistor 13 instead of the cross-coupled flip-flop 3 shown in FIG. 10A.
The logic constituting gates may be connected to or disconnected from each other by turning on or off the transistor 5 according to the switching data stored in the floating gate type of transistor 13. Since the data stored in the floating gate type of transistor 13 are also rewritable, connection or disconnection between the logic constituting gates may be changed for any number of times.
Therefore, using an FPGA provided with a number of switch circuits 1 shown in FIG. 10A or switch circuits 11 shown in FIG. 10B, it is possible to change the logic constitution after implementation for any number of times.
However, the above-described switch circuit 1 or 11 in the conventional arrangement has the following problems.
That is to say, since the cross-coupled flip-flop 3 of the switch circuit 1 is a volatile memory element, the switching data are lost when power supply is turned off. Therefore, the switching data must be written to the cross-coupled flip-flop 3 every time the power supply is turned on. To avoid such a procedure, it is necessary to take such measures as keeping on a backup power after turning off the main power.
On the other hand, since the floating gate type of transistor 13 of the switch circuit 11 is a nonvolatile memory element, the cumbersome procedure or measures as required of the switch circuit 1 are unnecessary.
However, to rewrite the switching data of the floating gate type of transistor 13, an operation voltage higher than that for reading is required. This in turn requires a special (programming) device for rewriting the switching data. Moreover, with the floating gate type of transistor 13, the writing speed is considerably slower than the reading speed. Furthermore, the rewritable number of times of the switching data to the floating gate type of transistor 13 is about 105−106 times, and is not so much.